1. Field of the Invention
This invention relates to epoxidized acetals and thioacetals, episulfidized acetals and thioacetals, thermosetting resin compositions based on such epoxidized acetals and thioacetals, episulfidized acetals and thioacetals, reaction products of which are controllably degradable when subjected to appropriate conditions.
2. Brief Description of Related Technology
In the field of microelectronic device assembly, the popularity of small-sized electronic appliances, such as camera-integrated video tape recorders (“VTRs”) and portable telephone sets, has made size reduction of large scale integration (“LSI”) devices desirable. As a result, chip size or chip scale packages (“CSPs”), ball grid arrays (“BGAs”), land grid arrays (“LGAs”) and the like, each of which having a semiconductor chip, such as LSI, on a carrier substrate are being used to reduce the size of packages substantially to that of bare chips. Such CSPs, BGAs and LGAs improve the characteristics of the electronic device while retaining many of their operating features, thus serving to protect semiconductor bare chips, such as LSIs, and facilitate testing thereof.
Ordinarily, the CSP/BGA/LGA assembly is connected to electrical conductors on a circuit board by use of a solder connection or the like. However, when the resulting CSP/BGA/LGA circuit board structure is exposed to thermal cycling, the reliability of the solder connection between the circuit board and the CSP/BGA/LGA often becomes suspect. Recently, after a CSP/BGA/LGA assembly is mounted on a circuit board, the space between the CSP/BGA/LGA assembly and the circuit board is often now filled with a sealing resin (often referred to as underfill sealing) in order to relieve stresses caused by thermal cycling, thereby improving heat shock properties and enhancing the reliability of the structure.
However, since thermosetting resins are typically used as the underfill sealing material, in the event of a failure after the CSP/BGA/LGA assembly is mounted on the circuit board, it is very difficult to replace the CSP/BGA/LGA assembly without destroying or scrapping the structure in its entirety.
To that end, techniques for mounting a bare chip on a circuit board are accepted as substantially similar to the mounting of a CSP/BGA/LGA assembly onto a-circuit board. One such technique, disclosed in Japanese Laid-Open Patent Publication No. 102343/93, involves a mounting process where a bare chip is fixed and connected to a circuit board by use of a photocurable adhesive, where, in the event of failure, this bare chip is removed therefrom. However, this technique is limited to those instances where the circuit board includes a transparent substrate (e.g., glass) which permits exposure to light from the back side, and the resulting structure exhibits poor heat shock properties.
Japanese Laid-Open Patent Publication No. 69280/94 discloses a process where a bare chip is fixed and connected to a substrate by use of a resin capable of hardening at a predetermined temperature. In the event of failure, this bare chip is removed from the substrate by softening the resin at a temperature higher than the predetermined temperature. However, no specific resin is disclosed, and there is no disclosure about treating the resin which remains on the substrate. Thus, the disclosed process is at best incomplete.
As pointed out in Japanese Laid-Open Patent Publication No. 77264/94, it is conventional to use a solvent to remove residual resin from a circuit board. However, swelling the resin with a solvent is a time consuming process and the corrosive organic acid ordinarily used as the solvent may reduce the reliability of the circuit board. Instead, that disclosure speaks to a method for removing residual resin by irradiation with electromagnetic radiation.
Japanese Laid-Open Patent Publication No. 251516/93 also discloses a mounting process using bisphenol A type epoxy resin (CV5183 or CV5183S; manufactured by Matsushita Electric Industrial Co., Ltd.). However, the removal process so disclosed does not consistently permit easy removal of the chip, the curing step is lengthy at elevated temperatures, and the process generally results in poor productivity.
Of course, mechanical methods of removing/replacing semiconductor chips from/on a substrate are known, such as by cutting the chip to be removed/replaced. See U.S. Pat. No. 5,355,580 (Tsukada).
Thermoplastic underfill resins are known for use in semiconductor chip attachment. See U.S. Pat. No. 5,783,867 (Belke, Jr.). However, such thermoplastic resins tend to leak under relatively modest temperature conditions. In contrast, thermosetting resins cure into a matrix which ordinarily have greater thermal stability under end use operating temperatures.
U.S. Pat. No. 5,760,337 (Iyer) refers to thermally reworkable crosslinked resins to fill the gap created between a semiconductor device and a substrate to which it is attached. These resins are produced by reacting a dienophile (with a functionality greater than 1) with a 2.5-dialkyl substituted furan-containing polymer.
International Patent Publication No. PCT/US98/00858 refers to a thermosetting resin composition capable of sealing underfilling between a semiconductor device including a semiconductor chip mounted on a carrier substrate and a circuit board to which said semiconductor device is electrically connected. The composition includes about 100 parts by weight of an epoxy resin, about 3 to about 60 parts by weight of a curing agent and about 1 to about 90 parts by weight of a plasticizer. Here, the area around the cured thermoset is to be heated at a temperature of about 190 to about 260° C. for a period of time ranging from about 10 seconds to about 1 minute in order to achieve softening and a loss of much of its adhesiveness.
Recent commercial interest has led to industry efforts to produce thermoset underfill sealants, which are capable of being reworked at temperatures slightly greater than those ordinarily used, particularly where the underfill sealant compositions have been designed to cure during the solder reflow cycle. To date, apart from the present invention, it is not believed that such an underfill sealant exists or has been evaluated.
Several classes of labile-group functionalized diepoxide monomers have been examined as reactive components of reworkable underfill compositions for the bonding and reinforcement of solder-bumped flip-chip attachments to circuit boards. These compositions cure under exposure to elevated temperature conditions to form network structures that are intended to mechanically stabilize the soldered devices.
In addition, the compositions oftentimes are prepared from curable resins having a thermally labile or readily hydrolyzable functionality that permits the network to be readily degraded when the device is heated at elevated temperatures and/or when it is exposed to an acidic solution. The labile groups that have been used to impart reworkable properties to the epoxy adhesive include secondary and tertiary esters [U.S. Pat. Nos. 5,948,922 (Ober) and 5,973,033 (Ober), each of which refer to a certain class of compounds having tertiary oxycarbonyl linkages, and compositions based on such compounds, which when cured provide thermally decomposable compositions capable of being reworked; S. Yang et al., Chem. Mater., 10(6), 1475 (1998), J. S. Chen et al., ACS Polymer Preprints, 41(2), 1842 (2000), H. Li et al., ACS PMSE Preprints, 83, 563 (2000)], aliphatic acetals [U.S. Pat. Nos. 5,512,613 (Afzali-Ardakani), 5,560,934 (Afzali-Ardakani) and 5,932,682 (Buchwalter), each of which refer to a reworkable thermoset composition based on a diepoxide component in which the organic linking moiety connecting the two epoxy groups of the diepoxide includes an acid cleavable acyclic acetal group, in combination with an anhydride and a diaza compound (such as an imidizole) together with a hydroxy initiator; U.S. Pat. No. 6,008,266, and S. Buchwalter et al., ACS PMSE Preprints, 72, 450 (1995)]; acetal diacrylates [U.S. Pat. No. 5,872,158 (Kuczynski) refers to thermosetting compositions capable of curing upon exposure to actinic radiation, which are based on acetal diacrylates, and reaction products of which are reported to be soluble in dilute acid]; and various carbamates [L. Wang and C. Wong, J. Polym. Sci. Part A, 37, 2991 (1997)].
While many of these compositions provide rapid degradation under certain conditions, commercial obstacles exist to their wide use. For instance, epoxidized secondary and tertiary esters are costly to produce and generally undergo network degradation at lower temperatures than would be desirable for widespread application. Epoxidized aliphatic acetals decompose very slowly at high temperatures, and generally require the introduction of acidic solutions to accelerate the decomposition. The acidic solution adds a considerable inconvenience and cost to the process, in addition to increasing the cost. Epoxidized carbamates ate also costly to produce; more importantly, however, they have the added undesirable effect of yielding highly toxic isocyanates following thermolysis. In addition, these compositions ordinarily leave decomposition residues on the circuit board, which the circuit board assembler needs to remove, thereby rendering the replacement of the semicondutor more difficult than is desirable.
Thus, there is a need for epoxy-based adhesive compositions that among other things (1) decompose rapidly at relatively high temperatures but not so high that the elevated temperature conditions may compromise the integrity of substrates, (2) are inexpensive to produce, (3) do not generate toxic by-products on decomposition and (4) to the extent they leave residues, such residues are easy to clean.